The detection of explosives on different environments is of interest for Homeland Security and National Defense. In the last few years, Surface enhanced Raman scattering (SERS) has emerged as an important technique for the detection of biological and energetic materials due to its high sensitivity. SERS is a powerful spectroscopic technique for detecting low concentrations of analytes. Since its observation was first observed in 1974 and correctly explained in 1977 it has overcome the extremely low cross sections (10−31 cm2) of conventional Raman Spectroscopy by enhancing the typically low Raman intensity with factors of 106 to 1014. The vibrational spectrum of a molecule displays a fingerprint map of the chemical composition of each chemical or biological agent. However the application of SERS for detecting nitroexplosives is still a challenging application to overcome due to the low affinity of these analytes to nanometallic surfaces. It was demonstrated that when the detection of 2,4,6-TNT was conducted with colloidal silver and gold in aqueous solutions a low limit of detection of 10−7 M could be achieved under optimum conditions. Gold, silver, and Au/Ag colloids synthesized by chemical reduction methods have also been used for detecting 2,4,6-TNT and 2,4-DNT in solution with high sensitivity and molecular specificity. In this application pH changes were needed to improve the detection. This proposed method has the problem that is an indirect detection which involves the alkaline hydrolysis of the explosives by causing the degradation of the explosives.
CTAB-gold nanoparticles have been used for other nitro explosives such as RDX with a wide nanometer size. Although the sensitivity of the nanoparticles was in the picogram range, the procedure needed the mixture of the colloidal suspension with the explosive before the deposition of them on a glass slide.
The detection of TNT, TATP and NG onto solid SERS substrates have been reported by depositing a gold layer on a silicon surface formed by microlithography. However, the sensitivity was in the range of 200-400 pg and high acquisition (signal integration) time had to be employed in the case of TNT (600 s).
A major problem related with the use of metal colloids is the tendency for colloidal aggregation after the addition of analytes, which makes the colloid unstable and leads often to poor reproducibility of the SERS spectra. To avoid variations in the SERS enhancement due to changes in colloid aggregation, a vast number of SERS investigations employ solid support-based substrates, such as metal island films and electrochemically roughened metals. Other techniques for nanostructure fabrication that can give better reproducibility than colloidal suspensions include electron beam lithography (EBL) and nanosphere lithography (NSL) provide fine control over interparticle spacing and can be used to generate nanostructures of different shapes. However, they constitute expensive and slow methods of preparation. Other shapes such as nanorods, nanowires and nanocubes are few of the reported shapes in the literature as SERS active substrates that have sharp surface features that produce vast localized field enhancements at the tips of the sharp features that make them attractive as good substrates for SERS applications. Gold-cetyl trimethylammonium bromide (CTAB) nanoparticles have been synthesized and used for the detection of 4-mercaptobenzoic acid on immobilized nanoparticles in a sandwich form in which the analyte is between a gold slide and the nanoparticles. This method works for analytes that can form a chemical bond with the gold slides and that can have an electrostatic interaction with CTAB-gold nanoparticles in the way to ease their attachment on it. The disadvantage of this method as a sensor for explosives is the fact that the nanoparticles are added to the gold slide after attaching the analyte and nitroexplosives are characterized for having poor affinity for the gold slide compared to sulfur compounds and the analysis would take a long time to be efficient in sensors applications. Also while trying to bind explosives to nanoparticle deposition at high solvent temperature would cause the evaporation of the nitroaromatic explosives from the solution due low water solubility and vapor pressure of many of these explosives.
The synthesis of gold nanorods is always accompanied by the formation of other shapes in the solution and their removal is a grand challenge. Platelets can be dissolved by adding Au(III)/CTAB complex to the nanorod final solution resulting in a conversion of platelets into smooth disk like structures, but the process takes hours for completion. A centrifugation protocol developed used to separate other shapes from nanorod solution is described which takes ˜21 min.
What is needed is an inexpensive and versatile methodology of preparation, colloidal aggregation, high signal reproducibility (regarding SERS vibrational signals), sensitivity, applications to gas phase and while searching for sensing methodologies that would be able to detect nanomoles to femtomoles of high explosives on solid support SERS substrates.